Charles Vacca was standing next to the girl when she squeezed the trigger at the Last Stop range in White Hills, Arizona, on Monday. The recoil wrenched the Uzi upward, and Vacca was fatally shot once in the head.

His ex-wife, Anamarie Vacca, says she hasn’t spoken to the girl or her parents since the shooting occurred, but that her children want to write the letter -- and let the girl know ‘‘not to revolve her life around it.’’

Groups seeking to reduce gun violence have said it was reckless to let the girl handle such a powerful weapon. They’re calling for tighter regulations regarding children and guns.